In protoplanetary disks, planetesimals are formed through the gravitational instability or simple coalescence. If the gravitational instability occurs, it needs that dust aggregates becomes large aggregates (a few m) that gas drag force is not efficient. Small dust aggregates have the fluffy structure. However, dust aggregates are compressed before it becomes large. Thus, large dust aggregates can not have such fluffy structure. Such compression change the cross section. The gas drag force which govern the dust motion is proportional to cross section. Thus, when and how to occur compression is important on dust growth. In this study, we perform the N-body simulation of aggregate collisions. We repeat collisions between two aggregates. The initial aggregate is the aggregate obtained in the preceding collision. In our simulation, collisions between dust which have a variety of the impact velocity occur. We examined the density change at such collisions. We introduce the pressure and construct the model which is consistent with our numerical simulations.
